This invention concerns hoists and more particularly fluid pressure actuated hoists in which a load cable is wound on a spool drum.
One form of such hoists commonly used in industrial plants is a pneumatic hoists operated with "shop air" commonly piped through the plant for use in operating various equipment used in manufacturing operations.
A particularly advantageous design is the air balancing hoists such as described in U.S. Pat. No. 3,428,298 issued on Feb. 18, 1969 for a "Tool Balancer." These hoists use a ball screw mechanism operated by a large diameter piston defining an air chamber which is advanced axially to push a ball nut mounted on a fixed ball screw. The ball nut is attached to a spool drum which is rotated as the nut advances axially. By precisely regulating the air pressure in the air chamber to an adjusted value, the weight of a load supported on a cable wound on a spool drum can be balanced. By increasing or decreasing the pressure, the cable is wound or unwound to raise or lower the load.
In such industrial plants, it is sometimes desirable to provide portable units to perform moderate lifting tasks at locations where an overhead hoist is not installed, and a connection to an air line is not practical. A self-contained fluid power source is thus provided, such portable fluid power sources are preferably battery-powered to avoid the need for wiring an electrical connection.
Battery power is used to generate fluid pressure to operate fluid power devices for lifting loads.
Battery-powered air compressors consume too much power for battery operation, and hence hydraulic power is used.
Power cylinders operated by hydraulic pressure are sometimes used to lift loads through a cable and pulley system, but the volume of hydraulic fluid required to operate the cylinders results in slow lifting speeds when limited by the battery operation and also results in rapid consumption of the available battery power. Also, the length of the cylinders required is excessive as the lifting distance is typically on the order of several feet.
Various displacement multipliers could be employed, such as the ball screw mechanism used in air balancing hoists, but again the volume of fluid typically required by integrated fluid pressure chambers is relatively large, resulting in slow lifting speeds and excessive battery power consumption.
The object of the present invention is to provide a hydraulic-powered lifting hoist which only requires minimal volume so as to be able to be operated at high speeds by a portable source of hydraulic fluid pressure.